rust-cpython: start cpython crate bindings This changeset introduces the hg-cpython crate, that compiles as a shared library holding a whole Python package (mercurial.rustext), with only the empty 'ancestor' submodule for now. Such bindings will be easier and safer to develop and maintain that those of `hg-direct-ffi`. They don't involve C code, only unsafe Rust that's mostly isolated within the cpython crate. The long-term goal would be to import the provided modules, such as rustext.ancestor with mercurial.policy.importmod, same as we already do with cext modules. Differential Revision: https://phab.mercurial-scm.org/D5434

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-cpython/src/exceptions.rs	Mon Dec 03 06:52:17 2018 +0100
@@ -0,0 +1,15 @@
+use cpython::exc::ValueError;
+use cpython::{PyErr, Python};
+use hg;
+
+py_exception!(rustext, GraphError, ValueError);
+
+impl GraphError {
+    pub fn pynew(py: Python, inner: hg::GraphError) -> PyErr {
+        match inner {
+            hg::GraphError::ParentOutOfRange(r) => {
+                GraphError::new(py, ("ParentOutOfRange", r))
+            }
+        }
+    }
+}